This invention relates to methods and systems for detecting the presence of a magnetic object within a predetermined zone. More particularly, the present invention relates to an improved method and apparatus for detecting the passage of an item having the magnetic object attached thereto through an interrogation zone.
In the past there have been many different systems proposed for use in detecting pilferage of items from an enclosed area by persons who normally have access to the enclosed area. Many anti-pilferage systems detect passage of an item through an exit point by monitoring for a signal emitted by a transponder attached to the item. A common anti-pilferage system generates a magnetic alternating field in an exit passageway through which all persons within the enclosed area must exit. A search is made for preselected harmonics that are generated by a high permeability magnetic tag attached to the item when the tag is within the alternating field. Another anti-pilferage system proposed in the past generates two different frequencies within the exit passage way and then monitors for sum and/or difference frequencies created when the high permeability marker or tag passes through the exit way. A simplified version of such a system is described in U.S. Pat. No. 3,631,442. The use of high permeability magnetic tags attached to items desirous of being protected from pilferage was taught in French Pat. No. 763,681 which issued to Pierre A. Picard.
One of the shortcomings with the systems of the past is the tendency to generate an alarm when an item bearing the tag is not present within the exit way or interrogation zone. Not only are these false alarm systems embarrassing to the personnel monitoring the theft detection system, but they can also subject the management of the enclosed area to a law suit by the individual subjected to questioning or accused of causing the alarm. Repeated false alarms eventually cause personnel monitoring the theft detection system to ignore all alarms whether false or not. It will therefore be appreciated that it would be desirable to have a theft detection or anti-pilferage system that does not generate false alarms.
The present invention relates to a system having greatly improved resistance, over prior art systems, to the production of false alarms caused both by passage through the interrogation zone of miscellaneous metallic items and by the presence of extraneous environmentally caused interference. This improved theft detection system also has greatly improved sensitivity to the marker tag employed so that improved detectability is obtained. The improved system allows detection of the marker tag to be made even in cases of simultaneous passage through the interrogation zone of larger sized competitive metallic objects which would inhibit previous systems from making such a detection. This improved system introduces new means for operating in the presence of large stationary metallic objects, such as underlying steel floor beams or metallic columns which would normally disrupt the proper operation of prior systems. The improved system employs low amplitude audio frequency magnetic fields in the interrogation zone.
Accordingly, one of the objects of the present invention is to provide an improved theft detection system which has a greatly improved resistance to the production of false alarms.
Another object of the invention is to provide a theft detection system which generates two different frequencies within an interrogation zone and senses a ratio of sidebands generated from the two frequencies by a tag within the interrogation zone.
A further object of the present invention is to provide a receiver portion of a theft detection system which operates upon signals in a manner to essentially eliminate false alarms.
Yet another object of the present invention is to provide an improved method of monitoring an interrogation zone to determine the presence of a tag within the zone and uniquely processing a sensed signal in a manner to eliminate false alarms.